refactor: Give internal linkage to global mutex

The `static` keyword was removed by #8674 but I think it should be
restored so that this global can't clash with anything else called
`::s_mutex` elsewhere in the executable.

src/lib/arch/ArchString.cpp

@@ -12,7 +12,7 @@
 #include <cstring>
 #include <mutex>
 
-std::mutex s_mutex;
+static std::mutex s_mutex;
 
 //
 // use C library non-reentrant multibyte conversion with mutex